U.S. patent number 6,262,827 [Application Number 09/559,093] was granted by the patent office on 2001-07-17 for galvano-mirror.
This patent grant is currently assigned to Fujitsu Limited. Invention is credited to Hisao Okuda, Satoshi Ueda.
United States Patent |
6,262,827 |
Ueda , et al. |
July 17, 2001 |
Galvano-mirror
Abstract
A galvano-mirror includes a mirror substrate provided with a
frame, a mirror element and torsion bars rotatably connecting the
mirror element to the frame. The mirror element is provided with a
body having an obverse surface and a reverse surface. A mirror
surface is formed on the obverse surface, while first electrodes
are formed on the reverse surface. The galvano-mirror also includes
an electrode substrate provided with second electrodes arranged in
facing relation to the first electrodes. The electrode substrate is
formed with a through-hole extending through a thickness thereof
and facing the mirror element.
Inventors: |
Ueda; Satoshi (Kawasaki,
JP), Okuda; Hisao (Kawasaki, JP) |
Assignee: |
Fujitsu Limited (Kawasaki,
JP)
|
Family
ID: |
16132448 |
Appl.
No.: |
09/559,093 |
Filed: |
April 27, 2000 |
Foreign Application Priority Data
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|
|
Jun 29, 1999 [JP] |
|
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11-183253 |
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Current U.S.
Class: |
359/224.1;
359/223.1 |
Current CPC
Class: |
G02B
26/0841 (20130101) |
Current International
Class: |
G02B
26/08 (20060101); G02B 026/08 () |
Field of
Search: |
;359/198,199,223,224 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
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|
|
5-119280 |
|
May 1993 |
|
JP |
|
9-146034 |
|
Jun 1997 |
|
JP |
|
Other References
IBM J. Res. Develop.--vol. 24, No. 5, Sep. 1980..
|
Primary Examiner: Phan; James
Attorney, Agent or Firm: Armstrong, Westerman, Hattori,
McLeland & Naughton LLP
Claims
What is claimed is:
1. A galvano-mirror comprising:
a mirror substrate including a frame, a mirror element and torsion
bars rotatably connecting the mirror element to the frame, the
mirror element being provided with a body having an obverse surface
and a reverse surface, the mirror element being also provided with
a mirror surface formed on the obverse surface and first electrodes
formed on the reverse surface; and
an electrode substrate provided with second electrodes arranged in
facing relation to the first electrodes;
wherein the electrode substrate is formed with a through-hole
extending through a thickness thereof and facing the mirror
element.
2. The galvano-mirror according to claim 1, wherein the second
electrodes are arranged adjacent to the through-hole.
3. The galvano-mirror according to claim 1, further comprising
stoppers for preventing the first electrodes from contacting with
the second electrodes.
4. A galvano-mirror comprising:
a mirror substrate including a frame, a mirror element and torsion
bars rotatably connecting the mirror element to the frame, the
mirror element being provided with a body having an obverse surface
and a reverse surface, the mirror element being also provided with
a mirror surface formed on the obverse surface and first electrodes
formed on the reverse surface;
an electrode substrate formed with second electrodes facing the
first electrodes;
third electrodes formed on the obverse surface of the mirror
element; and
a supporting structure provided with fourth electrodes facing the
third electrodes.
5. The galvano-mirror according to claim 4, wherein the electrode
substrate is formed with a through-hole extending through a
thickness thereof and facing the mirror element.
6. The galvano-mirror according to claim 5, wherein the
through-hole is generally symmetrical with respect to axes of the
torsion bars, the second electrodes comprising two conductive
layers which are arranged close to the through-hole and symmetrical
with respect to the axes of the torsion bars.
7. The galvano-mirror according to claim 6, wherein the
through-hole is rectangular.
8. The galvano-mirror according to claim 6, wherein the
through-hole is elliptic.
9. The galvano-mirror according to claim 6, wherein the
through-hole is circular.
10. The galvano-mirror according to claim 4, wherein the first
electrodes, the second electrodes, the third electrodes and the
fourth electrodes comprise two conductive layers, respectively,
which are arranged symmetrically with respect to axes of the
torsion bars.
11. The galvano-mirror according to claim 10, wherein the two
conductive layers of the first electrodes are electrically
connected to each other, the two conductive layers of the third
electrodes being electrically connected to each other, voltage
being simultaneously applied both to one of the two conductive
layers of the second electrodes and to one of the two conductive
layers of the fourth electrodes for generating a torque in the
mirror element.
12. The galvano-mirror according to claim 10, wherein the two
conductive layers of the second electrodes are electrically
connected to each other, the two conductive layers of the fourth
electrodes being electrically connected to each other, voltage
being simultaneously applied both to one of the two conductive
layers of the first electrodes and to one of the two conductive
layers of the third electrodes for generating a torque in the
mirror element.
13. The galvano-mirror according to claim 4, wherein an
electrostatic force generated by voltage application across the
first and the second electrodes has a first component parallel to
the mirror element, and an electrostatic force generated by voltage
application across the third and the fourth electrodes has a second
component parallel to the mirror element, the first component being
cancelled out by the second component.
14. The galvano-mirror according to claim 4, wherein the supporting
structure has a one-piece frame-like configuration.
15. The galvano-mirror according to claim 14, wherein the
supporting structure is formed with an opening for allowing passage
of light.
16. The galvano-mirror according to claim 4, wherein the supporting
structure comprises a pair of supporting members each of which has
an L-shaped cross section.
17. The galvano-mirror according to claim 4, further comprising
stoppers for restricting rotation of the mirror element to prevent
the first electrodes from contacting with the second electrodes and
to prevent the third electrodes from contacting with the fourth
electrodes.
18. The galvano-mirror according to claim 17, wherein the stoppers
protrude from the electrode substrate.
19. The galvano-mirror according to claim 4, wherein the first and
the third electrodes are electrically connected to each other.
20. The galvano-mirror according to claim 19, wherein the first and
the third electrodes are formed integral to each other.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electrostatically driven
galvano-mirror used in e.g. an optical disk apparatus for
controlling the direction of light emission.
2. Description of the Related Art
A typical electrostatically driven galvano-mirror is disclosed in
e.g. "Silicon Torsional Scanning Mirror (IBM J. RES. and DEVELOP.,
Vol. 24, No. 5, September 1980)". As shown in FIGS. 12 and 13 of
the accompanying drawings, the conventional galvano-mirror is
provided with a lower substrate 100 and an upper substrate 102
bonded to the lower substrate 100. The upper substrate 102 includes
a frame 104, a mirror element 106 (formed with a mirror surface
106a), and two torsion bars 108 connecting the mirror element 106
to the frame 104.
With such an arrangement, the mirror element 106 is deflected in a
torsional movement about the torsion bars 108 upon application of
external forces to the mirror element 106. The mirror element 106
has a bottom surface on which a pair of first electrodes 110a, 110b
are formed. Correspondingly, the lower substrate 100 is provided
with a pair of second electrodes 112a and 112b facing the first
electrodes 110a and 110b, respectively. The lower substrate 100 is
formed integrally with a ridge 100a contacting with the mirror
element 106. The ridge 100a extends along the aligned axes of the
torsion bars 108.
When voltage is applied across the first electrode 110a and the
second electrode 112a, the mirror element 106 is rotated
counterclockwise in FIG. 13 by electrostatic force. When voltage is
applied across the other first electrode 110b and the other second
electrode 112b, the mirror element 106 is rotated clockwise. Such
electrostatic force is proportional to the area of the respective
electrodes. Thus, for actuating the mirror element 106 with a low
voltage, the area of the first electrodes 110a, 110b needs to be
large, which may cause the electrodes 110a, 110b to cover almost
the entirety of the lower surface of the mirror element 106. The
size of the second electrodes 112a, 112b is determined in
correspondence to the size of the first electrodes 110a, 110b.
In the conventional galvano-mirror described above, the mirror
element 106 in motion tends to be subject to unfavorable damping
due to the viscosity of the air present between the mirror element
106 and the lower substrate 100. Consequently, it is difficult to
properly control the movement of the mirror element 106.
For reducing such viscous air-damping, the lower substrate 100 may
be formed with a plurality of grooves facing the mirror element
106, as taught in JP-A-9(1997)-146034 for example. However, the
additional processing of such grooves may make the fabrication
procedures of the galvano-mirror disadvantageously complex. As a
result, the production efficiency is lowered, while the cost is
unduly increased.
Another problem of the conventional galvano-mirror of FIGS. 12 and
13 is that the mirror element 106 may be displaced sideways upon
application of voltage across the first electrode 110a and the
second electrode 112a (or across the other first electrode 110b and
the other second electrode 112b). The mirror element 106 is moved
in this manner since the electrostatic force generated by the
voltage application has a horizontal component acting on the mirror
element 106. Such sideways displacement may render the posture of
the mirror element 106 unpredictable. Thus, desired control
accuracy in operating the mirror element 106 may be
unobtainable.
The above-described sideways displacement of the mirror element 106
may be reduced by attaching the mirror element 106 to the ridge
100a in a deflectable manner, as taught by JP-A-5(1993)-119280.
Specifically, the bottom surface of the mirror element 106 may be
formed with a groove into which the top of the ridge 100a is
received. The drawback of this arrangement is that the production
cost tends to be increased because it is necessary to precisely
form the groove in the mirror element 106 at the right position.
The ridge 100a also needs to be formed and positioned
accurately.
SUMMARY OF THE INVENTION
The present invention has been proposed under the circumstances
described above, and its object is to provide a galvano-mirror in
which the occurrence of viscous air-damping is reliably prevented.
The galvano-mirror is also advantageous in minimizing an increase
in production cost.
Another object of the present invention is to provide a
galvano-mirror in which the sideways displacement of the mirror
element is reliably prevented by taking inexpensive
countermeasures.
According to a first aspect of the present invention, there is
provided a galvano-mirror comprising:
a mirror substrate including a frame, a mirror element and torsion
bars rotatably connecting the mirror element to the frame, the
mirror element being provided with a body having an obverse surface
and a reverse surface, the mirror element being also provided with
a mirror surface formed on the obverse surface and first electrodes
formed on the reverse surface; and
an electrode substrate provided with second electrodes arranged in
facing relation to the first electrodes;
wherein the electrode substrate is formed with a through-hole
extending through a thickness thereof and facing the mirror
element.
With such an arrangement, the through-hole of the electrode
substrate permits freer flow of the air which would otherwise be
trapped between the mirror element and the electrode substrate.
Thus, the problem of the air-damping is advantageously
overcome.
In a preferred embodiment, the second electrodes are arranged
adjacent to the through-hole.
Preferably, the galvano-mirror may further comprise stoppers for
preventing the first electrodes from contacting with the second
electrodes.
According to a second aspect of the present invention, there is
provided a galvano-mirror comprising:
a mirror substrate including a frame, a mirror element and torsion
bars rotatably connecting the mirror element to the frame, the
mirror element being provided with a body having an obverse surface
and a reverse surface, the mirror element being also provided with
a mirror surface formed on the obverse surface and first electrodes
formed on the reverse surface;
an electrode substrate formed with second electrodes facing the
first electrodes;
third electrodes formed on the obverse surface of the mirror
element; and
a supporting structure provided with fourth electrodes facing the
third electrodes.
With such an arrangement, unfavorable sideways displacement of the
mirror element is effectively prevented by canceling out the
opposite components of the electrostatic forces acting on the
mirror element. In this manner, there is no need to accurately
process and position a ridge member to be formed in the electrode
substrate for holding the mirror element in place.
In the galvano-mirror of the second aspect again, the electrode
substrate may be formed with a through-hole extending through a
thickness thereof and facing the mirror element.
The above-mentioned through-hole may be generally symmetrical with
respect to axes of the torsion bars. Further, the second electrodes
may comprise two conductive layers which are arranged close to the
through-hole and symmetrical with respect to the axes of the
torsion bars.
Specifically, the through-hole may be rectangular, elliptic,
circular or the like.
Preferably, the first electrodes, the second electrodes, the third
electrodes and the fourth electrodes may comprise two conductive
layers, respectively, which are arranged symmetrically with respect
to axes of the torsion bars. This means that the first electrodes
may comprise two conductive layers symmetrical with respect to the
axes of the torsion bars, the second electrodes may also comprise
other two conductive layer symmetrical with respect to the axes of
the torsion bars, and so on.
In a preferred embodiment, the two conductive layers of the first
electrodes may be electrically connected to each other, and also
the two conductive layers of the third electrodes may be
electrically connected to each other. In this case, required
voltage may be simultaneously applied both to one of the two
conductive layers of the second electrodes and to one of the two
conductive layers of the fourth electrodes for generating a torque
in the mirror element.
Alternatively, the two conductive layers of the second electrodes
may be electrically connected to each other, and the two conductive
layers of the fourth electrodes may be electrically connected to
each other. In this case, required voltage may be simultaneously
applied both to one of the two conductive layers of the first
electrodes and to one of the two conductive layers of the third
electrodes for generating a torque in the mirror element.
Preferably, an electrostatic force generated by voltage application
across the first and the second electrodes may have a first
component parallel to the mirror element, and an electrostatic
force generated by voltage application across the third and the
fourth electrodes may have a second component parallel to the
mirror element. For preventing the sideways displacement of the
mirror element, the first component may be cancelled out by the
second component.
In a preferred embodiment, the supporting structure may have a
one-piece frame-like configuration.
Preferably, the supporting structure may be formed with an opening
for allowing passage of light.
In another preferred embodiment, the supporting structure may
comprise a pair of supporting members each of which has an L-shaped
cross section.
Preferably, the galvano-mirror may further comprise stoppers for
restricting rotation of the mirror element to prevent the first
electrodes from contacting with the second electrodes and to
prevent the third electrodes from contacting with the fourth
electrodes.
Preferably, the stoppers may protrude from the electrode
substrate.
In a preferred embodiment, the first and the third electrodes may
be electrically connected to each other. The first and the third
electrodes may be formed integral to each other.
Other features and advantages of the present invention will become
apparent from the detailed description given below with reference
to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an exploded perspective view showing a galvano-mirror
according to a first embodiment of the present invention;
FIG. 2A is a plan view showing a mirror element used for the
galvano-mirror of FIG. 1;
FIG. 2B is a front view showing the mirror element of FIG. 2A;
FIG. 3 is a sectional view showing a modified version of the
galvano-mirror of the first embodiment;
FIG. 4 is an exploded perspective view showing a galvano-mirror
according to a second embodiment of the present invention;
FIG. 5 is a sectional view showing a galvano-mirror according to a
third embodiment of the present invention;
FIG. 6 illustrates how electrostatic force acts on the mirror
element of the galvano-mirror of FIG. 1;
FIG. 7 illustrates how electrostatic force acts on the mirror
element of the galvano-mirror of FIG. 5;
FIGS. 8A-8K illustrate fabrication procedures of an upper substrate
(mirror substrate) used in the galvano-mirror of FIG. 5;
FIG. 9 is a sectional view showing a galvano-mirror according to a
fourth embodiment of the present invention;
FIG. 10 is a sectional view showing a galvano-mirror according to a
fifth embodiment of the present invention;
FIG. 11 is a plan view showing a conventional galvano-mirror;
and
FIG. 12 is a sectional view showing the conventional galvano-mirror
of FIG. 11.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The preferred embodiments of the present invention will be
described below with reference to the accompanying drawings.
Reference is first made to FIGS. 1, 2A and 2B illustrating a
galvano-mirror A1 according to a first embodiment of the present
invention. As best shown in FIG. 1, the mirror A1 includes a lower
substrate (electrode substrate) 2 and an upper substrate (mirror
substrate) 4 bonded to the lower substrate 2. The upper substrate
4, which may be made of e.g. silicon or aluminum, is provided with
a rectangular supporting frame 6, a rectangular mirror element 8,
and two torsion bars 10 connecting the mirror element 8 to the
frame 6. The axes 10a of the respective torsion bars 10 are aligned
(see FIG. 2A). The upper substrate 4 is symmetrical with respect to
the axes 10a of the torsion bars 10. With such an arrangement, the
mirror element 8 is deflectable in a torsional movement (or
rotatable) about the torsion bars 10 within a range of
predetermined angles.
As shown in FIG. 2B, the mirror element 8 has a planar body
provided with a lower (or reverse) surface 8a and an upper (or
obverse) surface 8b. A highly reflective layer or mirror surface 12
is formed on the upper surface 8b (see also FIG. 1). Two electrodes
(first electrodes) 14a and 14b are formed on the lower surface 8a
of the mirror element 8. As best shown in FIG. 2A, the first
electrodes 14a, 14b have the same rectangular configuration
extending in parallel to the torsion bars 10.
As shown in FIG. 1, the lower substrate 2 is formed with a
rectangular through-hole 16 facing the mirror element 8. The
through-hole 16 is symmetrical with respect to the axes 10a of the
torsion bars 10. Further, the lower substrate 2 is provided with
two electrodes (second electrodes) 18a and 18b flanking the
through-hole 16. The second electrodes 18a, 18b have substantially
the same configuration and are arranged symmetrically with respect
to the axes 10a of the torsion bars 10. The second electrode 18a
faces the first electrode 14a, while the other second electrode 18b
faces the other first electrode 14b.
The first electrodes 14a, 14b are electrically connected to each
other and grounded in common. The second electrodes 18a, 18b are
electrically insulated from each other and separately connected to
a controller (not shown). Though not illustrated, the first and the
second electrodes are coated with insulating layers.
As shown in FIG. 2A or 2B, the mirror surface 12 covers the
entirety of the upper surface 8b of the mirror element 8 except a
narrow, marginal region thereof. The first electrodes 14a, 14b,
which are formed on the bottom surface 8a of the mirror element 8,
are spaced from each other and located near opposite edges of the
bottom surface 8a. Thus, the first electrodes 14a, 14b extend in
parallel to the axes 10a of the torsion bars 10 and are arranged
symmetrically with respect to the axes 10a.
In the illustrated embodiment, the mirror element 8 has the same
length and width (2 mm) and a thickness of 300 .mu.m. The torsion
bars 10 have a length of 500 .mu.m, a width of 15 .mu.m and a
thickness of 50 .mu.m. When the mirror element 8 is in equilibrium
(i.e., not deflected about the torsion bars 10), the distance
between the mirror element 8 and the lower substrate 2 is 10 .mu.m.
The first electrodes 14a, 14b have a length of e.g. 2 mm and a
width of 150 .mu.m. Under control, the mirror element 8 is caused
to rotate about the axes of the torsion bars 10 through .+-.0.1
degrees.
The operation of the galvano-mirror A1 will now be described. As
stated above, the first electrodes 14a, 14b are grounded. Thus,
when positive or negative voltage is applied to e.g. the second
electrode 18a, the first and the second electrodes 14a and 18a are
electrostatically attracted to each other. Thus, the mirror element
8 is rotated counterclockwise (as viewed in FIG. 2B) about the
torsion bars 10. The angle of rotation of the mirror element 8 is
determined by the relation between the external torque acting on
the mirror element 8 and the resisting force of the torsion bars
10. Therefore, when the voltage applied to the second electrode 18a
becomes higher, the rotational angle of the mirror element 8 will
be greater. It follows that the rotational angle of the mirror
element 8 is controlled by varying the voltage applied to the
second electrode 18a.
As readily understood, it is possible to rotate the mirror element
8 clockwise about the torsion bars 10 by applying positive or
negative voltage to the other second electrode 18b.
As stated above, the distance between the mirror element 8 and the
lower substrate 2 is small (10 .mu.m). Thus, without taking
suitable countermeasures, the rotation of the mirror element 8
would be unduly damped due to the viscosity of the air to be
compressed or expanded between the mirror element 8 and the lower
substrate 2. In such an instance, it is difficult or even
impossible to control the rotational movement of the mirror element
8 with appropriate accuracy and quick response. The adverse effect
of the damping may become more conspicuous when the mirror element
8 needs to be operated at higher frequencies. The same problem may
result when the galvano-mirror A1 would be set in an atmosphere of
a gas other than the air.
According to the illustrated embodiment of the present invention,
however, the occurrence of damping is reliably prevented due to the
through-hole 16 formed in the lower substrate 2. This is because
the through-hole 16 permits freer flow of air which would otherwise
be trapped between the mirror element 8 and the lower substrate 2.
Preferably, the depth of the through-hole 16 (i.e., the thickness
of the lower substrate 2) is sufficiently greater than the distance
between the mirror element 8 and the lower substrate 2. In this
manner, undesirable damping of the mirror element 8 is effectively
prevented, even if the through-hole 16 is closed from below by e.g.
a supporting member on which the galvano-mirror A1 is mounted.
Due to the presence of the through-hole 16, the second electrodes
18a, 18b formed on the lower substrate 2 may need to be made
smaller than when such a through-hole is not provided. The
following section discusses how the rotational movement of the
mirror element 8 is affected by the size reduction in the second
electrodes 18a, 18b.
In general, when two flat electrodes are arranged in parallel to
each other, the electrostatic attraction F acting between them is
given by:
where .di-elect cons. is the dielectric constant of the medium
between the two electrodes, A is the area of each electrode, V is
the voltage applied to the electrodes and D is the distance between
the electrodes. The equation (1) shows that when the area (A) of an
electrode is reduced, the electrostatic attraction F acting on the
electrode is reduced proportionally.
It is also known that when an external force F is perpendicularly
applied to a pivotable bar, the torque T to be generated in the bar
is given by:
where L is the distance between the pivotal center and the action
point of the external force F.
As seen from the equation (2), the torque T exerted on the bar
becomes greater as the distance L increases.
According to the above equations (1) and (2), the following results
are obtained.
In the first embodiment, the presence of the through-hole 16
renders the second electrodes 18a, 18b comparatively small, and
accordingly the first electrodes 14a, 14b are also made small.
Consequently, as seen from the above equation (1), the dielectric
force generated between the first electrode 14a (or 14b) and the
second electrode 18a (or 18b) tends to be smaller than when the
through-hole 16 is not provided. Thus, the torque exerted on the
mirror element 8 will be smaller.
However, in the first embodiment, the above torque reduction is not
so large as to be detrimental to proper operation of the mirror
element 8. This is because the omitted parts of the second
electrodes 18a, 18b are closer to the axes 10a of the torsion bars
10 than the actually provided parts shown in FIG. 1. Therefore, an
additional torque which would be applied to the mirror element 8
without the through-hole 16 is advantageously small (equation (2)).
In the first embodiment, specifically, the torque reduction is only
less than 50%, while the area of each first electrode 14a, 14b is
reduced to 15%, as compared to the conventional galvano-mirror
shown in FIG. 12.
FIG. 3 shows a modified version of the galvano-mirror A1. The
modified galvano-mirror is identical in arrangement to the
galvano-mirror A1 except for stoppers 32a and 32b provided besides
the second electrodes 18a and 18b, respectively. With such an
arrangement, collisions between the first electrode 14a and the
second electrode 18a or between the other first electrode 14b and
the other second electrode 18b are reliably prevented from
occurring.
FIG. 4 shows a galvano-mirror A2 according to a second embodiment
of the present invention. In this figure, elements like or similar
to those of the first embodiment are designated by the same
reference numerals or signs as used for the first embodiment.
As illustrated in FIG. 4, the galvano-mirror A2 is basically
similar to the galvano-mirror A1 of the first embodiment except for
the through-hole 16 formed in the lower substrate 2. According to
the second embodiment, the through-hole 16 is circular. In this
way, the second electrodes 18a, 18b are made larger without
increasing the size of the lower substrate 2. Thus, it it possible
to increase the torque acting on the mirror element 8 or reduce the
voltage applied to the second electrodes 18a, 18b, without
increasing the overall size of the galvano-mirror A2. Instead of
the circular cross section, the through-hole 16 may have an
elliptic or other oval cross section.
FIG. 5 shows, in section, a galvano-mirror A3 according to a third
embodiment of the present invention. In this embodiment, third
electrodes 22a, 22b are formed on the upper surface of the mirror
element 8 in a manner such that the third electrodes 22a and 22b
are arranged opposite to the first electrodes 14a and 14b,
respectively.
The galvano-mirror A3 includes a supporting structure or top frame
28 mounted on the mirror substrate 4. The top frame 28 has a
generally rectangular, one-piece configuration and is formed with
an opening 26 for allowing the passage of light traveling toward or
away from the mirror surface 12. Fourth electrodes 24a, 24b
flanking the opening 26 are formed on the top frame 28. As
illustrated, the fourth electrodes 24a and 24b are held in facing
relation to the third electrodes 22a and 22b, respectively. The
third electrodes 22a, 22b are the same in size as the first
electrodes 14a, 14b. The fourth electrodes 24a and 24b are arranged
above the second electrodes 18a and 18b, respectively, with the
mirror element 8 intervening therebetween.
The two third electrodes 22a, 22b are electrically connected to
each other and grounded in common. The fourth electrodes 24a, 24b
are electrically insulated from each other, and separately
connected to a controller (not shown). The first to fourth
electrodes 14a-14b, 18a-18b, 22a-22b and 24a-24b are coated by
insulating layers (not shown).
In operation of the galvano-mirror A3, equal voltages are applied
to e.g. the first electrode 18a and the fourth electrode 24b.
Electrostatic attraction is then generated between the third
electrode 22b and the fourth electrode 24b, as well as between the
first electrode 14a and the second electrode 18a. The electrostatic
attraction (first attraction) acting on the first electrode 14a is
equal in strength but opposite in direction to the other
electrostatic attraction (second attraction) acting on the third
electrode 22b. Thus, the mirror element 8 is rotated
counterclockwise (in FIG. 5) about the torsion bars 10 by the
torque resulting from the first and the second attractions.
According to the third embodiment, two attractions of the same
strength are applied for rotating the mirror element 8, as state
above. In this manner, the torque generated in the mirror element 8
is made greater than when only one attraction is applied to the
mirror element 8 as in the galvano-mirror A1 of the first
embodiment. Specifically, the torque generated in the
galvano-mirror A3 may be double the torque generated in the
galvano-mirror A1 when the applied voltage is the same for the
mirrors A1 and A3.
Hence, for exerting a torque of the same magnitude on the mirror
elements 8 of the respective mirrors A1 and A3, a smaller voltage
may need to be applied to the mirror A3 as compared to the
galvano-mirror A1. Specifically, the voltage applied to the
galvano-mirror A3 may be (1/2) times the voltage applied to the
galvano-mirror A1.
Alternatively, the first to the fourth electrodes of the
galvano-mirror A3 may be smaller in area than the first and the
second electrodes of the galvano-mirror A1 for generating the same
torque in the mirror elements 8. Accordingly, the galvano-mirror A3
may be rendered smaller than the galvano-mirror A1. In this case,
the applied voltage may be the same for the mirrors A1 and A3.
The galvano-mirror A3 is also advantageous in the following
respect. For clarifying the additional advantage of the
galvano-mirror A3, comparison is made between the galvano-mirror A1
and the galvano-mirror A3 in reference to FIGS. 6 and 7.
When voltage is applied to the second electrode 18a of the mirror
A1, the mirror element 8 is rotated counterclockwise about the
torsion bars 10, as shown in FIG. 6. In this state, the
electrostatic force acting on the left edge of the mirror element 8
has an Fx-component parallel to the mirror element 8 and an
Fy-component perpendicular to the Fx-component. Because of the
Fx-component, the mirror element 8 of the mirror A1 may be unduly
moved to the left.
According to the third embodiment, on the other hand, electrostatic
forces will act on both the left and the right edges of the mirror
element 8 upon voltage application. As seen from FIG. 7, the right
and left Fx-components of the respective electrostatic forces are
cancelled out. Thus, the mirror element 8 of the galvano-mirror A3
does not undergo sideways displacement. Clearly, the same advantage
is obtainable when voltage is applied to the second electrode 18b
and the fourth electrode 24a.
FIGS. 8A through 8K show fabrication procedures of the mirror
substrate (upper substrate) 4 of the galvano-mirror A3 shown in
FIG. 5.
First, as shown in FIG. 8A, a wafer 52 made of silicon is
prepared.
Then, as shown in FIG. 8B, oxide layers 54a, 54b are formed over
the upper and the lower surfaces of the wafer 52.
Then, as shown in FIG. 8C, a predetermined pattern of a first
photoresist layer 56a is formed on the upper oxide layer 54a, while
a second photoresist layer 56b is formed over the entirety of the
lower oxide layer 54b. The pattern of the first photoresist layer
56a is configured for making the frame 6 (FIG. 5), the mirror
element 8 and the torsion bars (not shown). The exposed portions of
the first oxide layer 54a are etched away.
Then, as shown in FIG. 8D, the photoresist layers 56a and 56b are
removed. Thereafter, the wafer 52 is subjected to anisotropic
etching, so that grooves are formed in the wafer 52 at the
positions which are not covered by the upper oxide layer 54a. In
the illustrated example, the depth of the grooves may be 50 .mu.m,
which is equal to the thickness of the torsion bars.
Then, as shown in FIG. 8E, the oxide layers 54a and 54b are
stripped, and the wafer 52 is reoxidized to form an upper oxide
layer 58a and a lower oxide layer 58b over the upper and the lower
surfaces of the wafer 52, respectively.
Then, as shown in FIG. 8F, an upper metal layer 60a and a lower
metal layer 60b are formed over the upper oxide layer 58a and the
lower oxide layer 58b, respectively.
Then, as shown in FIG. 8G, a predetermined pattern of upper
photoresist layer 62a is formed on the upper metal layer 60a, while
a lower photoresist layer 62b is formed over the entirety of the
lower metal layer 60b. The pattern of the upper photoresist layer
62a is configured for making the mirror surface 12 (FIG. 5) and the
third electrodes 22a, 22b. The exposed portions of the upper metal
layer 60a are etched away. Thereafter, the upper and lower
photoresist layers 62a, 62b are removed.
Then, as shown in FIG. 8H, an upper photoresist layer 64a is formed
to cover the upper metal layer 60a, while a predetermined pattern
of a lower photoresist layer 64b is formed on the lower metal layer
60b. The pattern of the lower photoresist layer 64b is configured
for making the first electrodes 14a and 14b (FIG. 5). The exposed
portions of the lower metal layer 60b are etched away. Thereafter,
the upper and the lower photoresist layers 64a, 64b are
removed.
Then, as shown in FIG. 8I, an upper photoresist layer 66a is formed
to cover the upper oxide layer 58a and the upper metal layer 60a,
while a predetermined pattern of a lower photoresist layer 66b is
formed on the lower oxide layer 58b to cover the lower metal layer
60b. The pattern of the lower photoresist layer 66b is configured
for making the frame 6 (FIG. 5), the mirror element 8 and the
torsion bars (not shown). The exposed portions of the lower oxide
layer 58b are etched away. Thereafter, the upper and the lower
photoresist layers 66a, 66b are removed.
Then, as shown in FIG. 8J, the wafer 52 is subjected to anisotropic
etching, so that downwardly open grooves are formed in the wafer
52. As illustrated, the downwardly open grooves are deep enough to
reach the upper oxide layer 58a. At the time of this anisotropic
etching, a pair of torsion bars (not shown) are formed to connect
the inner piece, which corresponds to the mirror element 8, to the
outer piece surrounding the inner piece. The outer piece
corresponds to the frame 6 (see FIG. 5).
Finally, as shown in FIG. 8K, unnecessary portions of the upper
oxide layer 58a are etched away. Thus, the mirror substrate 4 of
the galvano-mirror A3 shown in FIG. 5 is obtained. As readily
understood, the upper metal layer 60a provides the mirror surface
12 and the third electrodes 22a, 22b, while the lower metal layer
60b provides the first electrodes 14a, 14b.
Reference is now made to FIG. 9 showing, in section, a
galvano-mirror A4 according to a fourth embodiment of the present
invention. Basically, the galvano-mirror A4 of this embodiment is
the same as the galvano-mirror A3 of the third embodiment (FIG. 5)
except that first and second stoppers 32a, 32b are integrally
formed on the lower substrate 2. Each of the stoppers 32a, 32b
projects upward from the lower substrate 2 and is elongated in
parallel to the axes of the non-illustrated torsion bars.
With such an arrangement, the first stopper 32a prevents the first
electrode 14a from colliding with the second electrode 18a, while
also preventing the third electrode 22b from colliding with the
fourth electrode 24b. Likewise, the second stopper 32b prevents the
first electrode 14b from colliding with the second electrode 18b,
while also preventing the third electrode 22a from colliding with
the fourth electrode 24a. Thus, the first to the fourth electrodes
are protected from mechanical damage or mutual adhesion which would
be caused without the stoppers 32a and 32b.
The design of the stoppers 32a, 32b is not limited to the
illustrated example. For instance, each of the stoppers 32a, 32b
may consist of a plurality of upward projections arranged in a row
extending along the axes of the torsion bars. Further, the stoppers
32a, 32b may be provided on the top frame 28 or mirror element
8.
FIG. 10 shows, in section, a galvano-mirror A5 according to a fifth
embodiment of the present invention. The galvano-mirror A5 is
provided with a supporting structure 28 consisting of two
supporting members. As illustrated, each supporting member 28
extends upright from the lower substrate 2 and then extends
horizontally. Thus, each supporting member 28 has an L-shaped cross
section. In this embodiment, the first electrode 14a is formed
integral with the third electrode 22a, while the other first
electrode 14b is formed integral with the other third electrode
22b. The supporting members 28 may be mounted on the frame 6 of the
mirror substrate 4, not on the lower substrate 2.
In the first to the fifth embodiments A1-A5 described above, only
one through-hole 16 is formed in the lower substrate 2.
Alternatively, more than one through-hole may be provided in the
lower substrate 2. Further, in these embodiments, the first
electrodes 14a-14b are grounded and voltage is applied to the
second electrodes 18a-18b. Alternatively, the second electrodes
18a-18b may be grounded, while voltage may be applied to the first
electrodes 14a-14b.
In the third to the fifth embodiments, the third electrodes 22a-22b
are grounded and voltage is applied to the fourth electrodes
24a-24b. Alternatively, the fourth electrodes 24a-24b may be
grounded, while voltage may be applied to the third electrodes
22a-22b.
Further, in the third to the fifth embodiments, the two of the
first electrodes 14a and 14b are symmetrical to each other with
respect to the axes of the torsion bars, and so are the respective
pairs of the second to the fourth electrodes 18a-18b, 22a-22b and
24a-24b. In this manner, the Fx-components of the electrostatic
forces acting on the mirror element 8 are cancelled out. However,
such an arrangement is exemplary, and the present invention is not
limited to this. Cancellation of the Fx-components of the
electrostatic forces may be achieved by adjusting the gaps between
the electrodes. In such an instance, the two of the first
electrodes 14a and 14b may differ in configuration and size from
each other. This may hold for the second to the fourth electrodes
18a-18b, 22a-22b and 24a-24b.
The present invention being thus described, it is obvious that the
same may be varied in many ways. Such variations are not to be
regarded as a departure from the spirit and scope of the present
invention, and all such modifications as would be obvious to those
skilled in the art are intended to be included within the scope of
the following claims.
* * * * *